The invention relates to a method for manufacturing thin-walled pipes, which pipes are made of a heat-resistant and wear-resistant aluminum-based material, in particular for use as cylinder liners for internal combustion engines.
Cylinder liners are components subject to wear, which are inserted, pressed or cast into the cylinder openings of the crankcase of the internal combustion engine.
The cylinder faces of an internal combustion engine are subjected to high frictional loads from the pistons or, respectively, from the piston rings and to locally occurring high temperatures. It is therefore necessary that these faces be made of wear-resistant and heat-resistant materials.
In order to achieve this goal, there are numerous processes amongst others to provide the face of the cylinder bore with wear-resistant coatings. Another possibility is to dispose a cylinder liner made of a wear-resistant material in the cylinder. Thus, gray-cast-iron cylinder liners were used, amongst others, which liners however exhibit a low heat conductivity as compared to aluminum-based materials and exhibit other disadvantages.
The problem was first solved with a cast cylinder block made of a hypereutectic aluminum-silicon AlSi alloy. The silicon content is limited to a maximum of 20 weight-percent for reasons associated with casting technology. As a further disadvantage of the casting method it is to be mentioned that primary silicon particles of relatively large dimensions (about 30-80 xcexcm.) are precipitated during the solidification of the melt. Based on the size and their angular and sharp-edged form, the primary silicon Si particles lead to wear at the piston and piston rings. One is therefore forced to protect the pistons and the piston rings with corresponding protective layers/coatings. The contact face of the silicon Si particles to the piston/piston ring is flat-smoothed through mechanical machining treatment. An electrochemical treatment then follows to such a mechanical treatment, whereby the aluminum matrix is slightly reset between the silicon Si grains such that the silicon Si grains protrude insignificantly as support structure from the cylinder face. The disadvantage of thus manufactured cylinder barrels lies, on the one hand, in a substantial manufacturing expenditure (costly alloy, expensive mechanical machining treatment, iron-coated pistons, armored and reinforced piston rings) and, on the other hand, in the defective distribution of the primary silicon Si particles. Thus, there are large areas in the microstructure which are free of silicon Si particles and thus are subject to an increased wear. In order to prevent this wear, a relatively thick oil film is required as separation medium between barrel and friction partner. The clearing depth of the silicon Si particles is amongst others decisive for the setting of the oil-film thickness. A relatively thick oil film leads to higher friction losses in the machine and to a larger increase of the pollutant emission.
In comparison, a cylinder block according to the DE 42 30 228, which is cast of an below-eutectic aluminum-silicon AlSi alloy and is provided with liners of a hypereutectic aluminum-silicon AlSi alloy material is more cost advantageous. However, the aforementioned problems are also not solved in this case.
In order to employ the advantages of the hypereutectic aluminum-silicon AlSi alloys as a liner material, the microstructure in regard to the silicon grains is to be changed. As is known, aluminum alloys, which cannot be realized using casting technology, can be custom-produced by powder-metallurgic processes or spray compacting.
Thus, in this way hypereutectic aluminum silicon AlSi alloys are produceable which have a very good wear resistance and receive the required heat resistance through alloying elements such, as for example iron Fe, nickel Ni, or manganese Mn, based on the high silicon content, the fineness of the silicon particles, and the homogeneous distribution. The primary silicon particles present in these alloys have a size of about 0.5 to 20 xcexcm. Therefore, the alloys produced in this way are suited for a liner material.
Even though aluminum alloys are in general easy to be processed, the deformation of these hypereutectic alloys is more problematic. A method for producing liners from a hypereutectic aluminum-silicon alloy is known from the German printed patent document EP 0 635 318. According to this reference, the liner is produced by extrusion presses at very high pressures and extrusion rates of 0.5 to 12 m/min. Very high extrusion rates are required in order to produce cost-effectively the liners to a final dimension with extruders. It has been shown that the high extrusion rates lead to a tearing of the profile during extrusion in case of such difficultly extrudable alloys and of the small wall thicknesses of the liners to be achieved.
The spray-compacting of hollow cylinders, the so-called tube blanks, is known from the WO 87/03012. For example, the manufacture of tube blanks having wall thicknesses of 25 to 40 mm is described. The same above-described problems occur upon deformation of such tube blanks to thin-walled pipes, for example, by extrusion.
The object of the invention is therefore to provide for an improved and much more cost-advantageous method for manufacturing thin-walled pipes, in particular for cylinder liners of internal combustion engines, wherein the finished liners are to exhibit the required property improvements in regard to wear resistance, heat resistance, and reduction of the pollutant emission.
The invention relates to a method for manufacturing thin-walled pipes, which are made of a heat-resistant and wear-resistant aluminum-based material.
The method comprises the spray-compacting of a thick-walled pipe made of a hypereutectic aluminum-silicon AlSi material, possibly a subsequent overaging annealing, and the hot deformation to a thin-walled pipe.
Such a method is in particular suited for the production of cylinder liners of internal combustion engines, since the produced liners exhibit the required properties in regard to wear resistance, heat resistance and reduction of pollutant emission.
Additional embodiments of the invention are given in the sub-claims.
The required tribological properties are in particular achieved in that silicon particles are present in the material as primary precipitates in a size range of from 0.5 to 20 xcexcm, or as added and admixed particles in a size range of up to 80 xcexcm. Methods have to be employed for the manufacture of such aluminum Al alloys which allow a substantially higher solidification rate of a high-alloy melt than it is possible with conventional casting processes.
On the one hand, the spray compacting method (in the following referred to as xe2x80x9cspray compactingxe2x80x9d) belongs to this. An aluminum alloy melt, highly alloyed with silicon, is atomized and cooled in the nitrogen stream at a cooling rate of 1000xc2x0 C./s. The in part still liquid powder particles are sprayed onto a support pipe, rotating horizontally around the longitudinal axis and made of the same type of material or a conventional aluminum material (for example, AlMgSi0.5). The support pipe, which has preferably wall thicknesses of from 2 to 3 mm, is linearly shifted under the spray beam during the process. By superpositioning the rotation motion and the translation motion of the support pipe, there is generated a cylindrical pipe having a fixed predetermined inner diameter. The outer diameter results from the charging speed and from the effective compacting rate. Pipes having wall thicknesses of from 6 to 20 mm can be manufactured in this way. A quasi continuous production operation can be achieved with suitable feed and guiding systems for the support pipes.
Primary silicon Si precipitates having a size of up to 20 xcexcm are generated in this spray-compacting process based on the high cooling speeds. An adaptation of the silicon Si precipitate size is achieved with the xe2x80x9cgas to metal ratioxe2x80x9d (standard cubic meter of gas per kilogram of melt), with which the solidification speed can be set in the process. Silicon Si contents of the alloys of up to 40 weight-% can be realized based on the high solidification speeds and the supersaturation of the melt. The supersaturation state in the resulting billet is quasi xe2x80x9cfrozenxe2x80x9d based on the fast quenching of the aluminum melt in the gas stream.
The spray compacting process in addition offers the possibility to enter particles with a particle injector into the billets or into the tube blanks, which particles were not present in the melt. There exists a plurality of adjustment possibilities for a microstructure since these particles can exhibit any desired geometry and any desired size between 2 xcexcm and 400 xcexcm. These particles can be, for example, silicon Si particles in the range of from 2 xcexcm to 400 xcexcm or oxide-ceramic particles (for example, Al2O3) or non-oxide-ceramic particles (for example, SiC, B4C, etc.) in the aforementioned particle-size spectrum, as they are commercially available and sensible for the tribological aspect.
The microstructural condition of the spray-compacted pipe can be changed with subsequent averaging annealing processes. The microstructure can be set with an annealing to a silicon grain size of from 2 to 30 xcexcm as it is desired for the required tribological properties. The growing of larger silicon Si particles during the annealing process is effected by diffusion in the solid at the expense of smaller silicon particles. This diffusion is dependent on the overaging and annealing temperature and the duration of the annealing treatment. The higher the temperature is chosen, the faster the silicon Si grains grow. Suitable temperatures are at about 500xc2x0 C., wherein an annealing duration of 3 to 5 hours is sufficient.
The thereby resulting and therefore custom-made microstructure no longer changes in the subsequent processing steps or it changes in the sense of the required tribological properties.
A reduction of the wall thickness to the required final dimensions is achieved by hot deformation by means of various processes dependent on the starting wall thickness of the such manufactured pipes. The process temperatures are between 300xc2x0 C. and 550xc2x0 C. In this case, the hot deformation serves not only for the forming, but also to the closing of the process-caused residual porosity (1-5%) in the spray-compacted starting material.
The pipe, formed to the final wall thickness, is subsequently cut into pipe sections of the required length.
The invention method has the advantage that the material for the liner can be custom-made. At the same time, the high expenditure in the case of one-step extruding of thin-walled pipes, both in regard to extrusion pressure and extrusion rate, as well as product quality and production economy, is successfully avoided based on the described method of production.